The present invention relates to the disposal of arsenic-contaminated waste material and more particularly relates to a method for encapsulating arsenic-contaminated waste material whereby it can be used as a permanent landfill material without the risk of the arsenic compounds later being leached into contacting ground waters.
In many known commercial processes, a waste material is generated which contains substantial amounts of arsenic and/or arsenic compounds. For example, synthetic hydrocarbons obtained from coal, oil shale, tar sands, or the like, normally contain substantial quantities of arsenic and/or arsenic compounds which have to be removed before the hydrocabons can be marketed. One process for treating such hydrocarbons is disclosed and fully described in U.S. Pat. No. 4,051,022, issued Sept. 27, 1977, and involves contacting the hydrocarbons with a guard bed material comprising a metal or combination of metals (e.g. iron, cobalt, nickel) and the oxides or preferably sulfides of said metals. The metal or metal compounds can be used by themselves or can be combined with a conventional support (carrier) e.g. silica, as understood in the art. Upon contact under prescribed conditions, the guard bed material removes and retains the arsenic from the feed hydrocarbons.
When the guard bed material becomes spent, a problem arises in properly disposing of same. The spent material cannot be used directly as a landfill material since it has been found that the arsenic and/or arsenic compounds will readily leach from the spent waste material when contacted by ground waters associated by the landfill site. Since this is ecologically unsound, a disposal process is necessitated which will prevent any substantial leaching of the arsenic from the spent material once it has been used as a landfill material.
Known disposal processes for waste materials of the type described above involve encapsulating the waste material into a concrete substrate and using the finished concrete as a landfill material. For example, both U.S. Pat. Nos. 3,980,558 and 4,142,192 disclose processes wherein arsenic contaminated wastes are mixed with cement and various additives to form a concrete landfill material. However, both references teach that Portland cement is the preferred cement for use in the disclosed processes.
Unfortunately, spent guard bed material of the type discussed above contains sulfur or sulfur compounds, in addition to arsenic. This is due to the original composition of the guard bed and/or from sulfur compounds, e.g. H.sub.2 S, which may be collected by the guard bed material from the hydrocarbon feed stock during treatment. Portland cement contains substantial amounts of tricalcium aluminate which react with the sulfur compounds, i.e. sulfates, in the spent guard bed materials to form ettringite which, in turn, expands upon as it forms and this causes the concrete to crack and disintegrate thereby effectively destroying the structural integrity of the concrete. Since ettringite may take a relatively long time (e.g. one year) to form, a concrete mass which appears structurally sound at the time of disposal may effectively disintegrate at the end of a prolonged period thereby allowing the arsenic encapsulated in the concrete to become exposed to leaching upon contact with water flowing through the disintegrated concrete. Further, if the ground water itself, (e.g. water containing gypsum) contains sulfur compounds, it may react with the tricalcium aluminate of the Portland cement with the same results.
Accordingly, where the encapsulated waste material contains sulfur and/or sulfur compounds or is likely to be in contact with waters which contain sulfur or sulfur compounds, Portland cement appears to be undesirable as an encapsulating material especially where the encapsulated material is used as a permanent landfill material which is likely to come into contact with ground waters.